I have to second that the Owls book, Mastering Regular Expressions, by Jeffery Friedl, published by O'Reilly & Associates is going to be your best bet. It doesn't dive into the actual code of Perl's RE engine, but it does offer a good discussion of DFA and NFA differences and attributes. And there's nowhere (including the POD) where you'll find a more thorough discussion of Perl's regular expression engine. Friedl himself does have a very thorough understanding of the code behind Perl's regular expression engine. However, if you have a specific need, you might try contacting one of the P5P individuals.